1. Field of the Invention
The present invention relates to the technical field of optical elements, imaging optical systems and imaging apparatuses, specifically to the technical field in which the spectral characteristics of incident light on an imaging device are adjusted to desirably improve characteristics such as color reproducibility in the red region.
2. Description of the Related Art
There have been recent demands for miniaturization of imaging apparatuses, such as digital video cameras and digital still cameras, with maintained image quality for pictures and videos.
To meet such demands, imaging apparatuses have been proposed that include a miniaturized imaging optical system, and a high-density CCD (Charge Coupled Device) or a high-density CMOS (Complementary Metal-Oxide Semiconductor) installed as an imaging device.
Generally, a number of techniques are known that realize high resolution to improve image quality in imaging optical systems that use an imaging device. Aside from high resolution, improving image quality involves another important factor—desirable color reproducibility for pictures and videos. The success or failure to ensure desirable color reproducibility is greatly influenced by the spectral characteristics of the optical element disposed on the light path.
For example, in one imaging apparatus of related art, an optical element having an infrared absorbing effect is disposed on the light path of an imaging optical system (see, for example, JP-A-2004-345680). In the imaging apparatus provided with such an optical element, desirable spectral characteristics need to be ensured for the optical element.
In response to the movement toward miniaturization of the imaging optical system or the lens barrel that houses the imaging optical system, there is an increasing tendency of the reflection ghost to occur by the reflection of light at the optical component of the lens barrel, particularly at the optical element including an multilayered film that interferes with ultraviolet rays and infrared rays. Suppression of the reflection ghost is therefore important to realize high image quality and miniaturization at the same time.
FIG. 10 to FIG. 12 are graphical representations of the spectral characteristics of an optical element of related art. In each figure, the upper graph represents the relationship between wavelength and spectral transmittance, and the lower graph represents the relationship between wavelength and the spectral reflectivity on each surface. In the lower graph, the symbols A and B denote the object-side surface and the image-side surface of the optical element, respectively.
FIG. 10 represents measurement values for the optical element that includes a base material formed of a clear glass plate, a spectra adjusting multilayered film formed on the object-side surface of the base material, and an antireflective film formed on the image-side surface of the base material.
FIG. 11 and FIG. 12 represent measurement values for two types of optical elements that include a base material formed of an infrared absorbing glass, a spectra adjusting multilayered film formed on the object-side surface of the base material, and an antireflective film formed on the image-side surface of the base material.
As represented in FIG. 10, the spectral transmittance abruptly varies near 650 nm in the optical element that uses a clear glass plate for the base material, because the base material does not have an infrared absorbing effect. Thus, unlike the optical elements represented in FIG. 11 and FIG. 12, unnecessary light is incident on the imaging device.
It is known that the wavelengths of light that tend to contribute to red reflection ghost are from about 600 nm to about 680 nm. The spectral reflectivity is high in this wavelength region in all of the optical elements represented in FIG. 10 to FIG. 12, and the red reflection ghost is likely to occur.